U.S. Pat. No. 4,419,558 to Stiebel et al. and an SAE Technical Paper entitled “Monitoring and Control of Spot Weld Operations” by Stiebel et al. describe monitoring and controlling electrical resistance spot-welding by measuring displacements of the electrodes during welding. After the squeezing force is applied by the electrodes to the work pieces and the supply of welding current is initiated, the metal at the work site first expands thermally as it heats (expansion) and then flows plastically as it softens and fuses (indentation). The electrodes are displaced by the expansion and indentation of the metal at the weld site as well as by the expansion and contraction of the electrodes. Thus, measurements of the displacement of the electrodes during formation of the weld provide information indicative of the state of the metal at the weld site.
It has long been known that moderate indentation almost always ensures a good weld. The ability to measure the onset of indentation makes it possible, therefore, to shut off the welding current upon detection of indentation with a high level of assurance that a good weld has been formed. The Stiebel et al. patent and the Stiebel et al. technical paper referred to above are incorporated in their entirety by the foregoing reference to them into the present specification.
In the method and apparatus of the Stiebel patent (and the Stiebel et al. technical paper) consistent measurements of displacement are assured by interposing a mechanical compression spring between the piston of an air cylinder (or its equivalent) that moves the movable electrode into engagement with the work piece and a stationary electrode.
A load cell associated with the spring detects the changes in the load imposed on the spring as the movable electrode is displaced upon expansion and indentation of the metal of the work pieces at the weld site during formation of the weld. Compressing the spring during expansion provides changes in the resulting forces in the spring and thus on the load cells that are directly proportional to the displacement of the movable electrode. Without the spring, for example with a hydraulic or pneumatic cylinder directly working on the movable electrode, the piston is theoretically free to displace with the movable electrode in direct correspondence with the electrode movements, thus providing no change in load and no opportunity to detect electrode displacements by detection of load changes.
U.S. Pat. No. 5,504,297, also to Steibel et al., applied these teachings by providing a pair of opposed electrodes mounted on arms that are fixed against outward displacement during the application of the welding current. One of the arms includes a pneumatic cylinder for moving one of the electrodes to and away from the workpiece. The cylinder includes a piston rod attached to a clutch by a ball screw mechanism. A brake assembly is connected to the unidirectional clutch to prevent outward displacement of the electrodes during expansion of the weld nugget and to permit inward movement of the electrodes to provide indentation of the weld after the nugget softens.
U.S. Pat. No. 5,789,719 to Pary et al. discloses an apparatus for electrical resistance spot welding that includes a pair of opposed electrodes wherein one electrode is movable into contact with a workpiece by actuating a hydraulic cylinder. A pilot operated check valve is provided that permits a one way flow of hydraulic fluid while the weld current is applied. The check valve is operative to prevent movement of the electrode away from the workpiece during expansion of the weld nugget, however, permits inward movement of the electrode after the weld nugget is softened.